It has been known for some time that rare-earth ion doped glasses in fiber form could be used as a lasing medium. However, it has only been recently that the possibility of using such fiber as the amplification medium in an optical fiber communication system has begun to be explored seriously. Most interest is currently directed towards fiber that comprises Er.sup.3+ ions. Among possible pump wavelengths (.lambda..sub.p) are those in the 0.8-1.0 .mu.m range (e.g., 0.98 .mu.m) and those relatively close to (but below) the anticipated signal radiation wavelength (.lambda..sub.s) of about 1.5 .mu.m (e.g., 1.48 .mu.m). See, for instance, P. Urquhart, IEE Proceedings, Vol. 135 (Pt.J, No. 6) pp. 385-407, December 1988 and the interview with E. Desurvire in Lasers and Optronics, May 1990, pp. 55-63.
The principle of amplification of an optical signal in an Er-doped fiber amplifier (EDFA) is known to those skilled in the art. See, for instance, J. R. Armitage, Applied Optics, Vol. 27(23), pp. 4831-4836, December 1988. However, in order to be of commercial interest, a fiber design has to be found that has certain desirable characteristics. Among these typically are high efficiency, low noise, low loss and acceptable mechanical properties (e.g., strength). In some cases it is also desirable for the fiber to have high saturation power. In other cases the fiber desirably has a low amplification threshold. By this is meant that net signal amplification can occur at relatively low pump power. Exemplarily, low amplification threshold is an important property for fiber that is to be used in a remotely pumped EDFA. Such fiber can, of course, also be used advantageously in other types of EDFA, e.g., locally pumped EDFA, including systems in which more than one pump laser feeds a length of amplifier fiber.
Several design criteria are known to those skilled in the art. For instance, J. R. Armitage (op. cit.) teaches that
i) the fundamental pump mode (LP.sub.01) is the optimum mode with which to pump the EDFA; and PA1 ii) the preferred dopant (i.e., Er.sup.3+) profile is one in which the dopant ions are confined just to the central region of the fiber core.
That author also states that, for a given form of the Er-profile, there exists a core size with which to achieve maximum amplification. Generally the optimum core provides for maximum overlap of the pump and signal radiation modes. Cores having effective diameter a and effective refractive index difference .DELTA.n (to be defined below) that result in a V-number at .lambda..sub.p of approximately 3 were stated to be optimal for use in a high gain EDFA.
However, even though at least some design criteria are known, in the design of an actual fiber amplifier trade-offs are frequently necessary. For instance, in practice it may not be possible to produce fiber that meets all the criteria. J. R. Armitage (op. cit.) states as follows: "In practice, control of the dopant profile, particularly for small core sizes, may prove to be difficult due to problems of dopant ion diffusion during the fiber making process. This may then impose a practical limit on how near to optimum it is possible to produce a real fiber."
In co-assigned U.S. patent application Ser. No. 467,699, incorporated herein by reference, fiber having improved operating characteristics (especially with regard to gain characteristics) is disclosed. In particular, the desirability of a relatively large (.about.0.03) refractive index difference between core and cladding is disclosed. In a, also co-assigned, continuation-in-part application of said application, even higher index differences are disclosed. This is a departure from the prior art, which generally uses fibers with considerably lower index differences. See, for instance, J. F. Massicott et al., Electronics Letters, Vol. 26(14), 1990, pp. 1038-1039, (.DELTA.n.about.0.013), and N. Kagi et al., Program of the Optical Fiber Communication Conference, San Francisco, 22-26 January 1990, 1990 Technical Digest Series, Vol. 1, Conference Edition, paper FA8 (numerical aperture 0.175, corresponding to .DELTA.n.about.0.0105).
It is also known that SiO.sub.2 -based fibers with an Al.sub.2 O.sub.3 --GeO.sub.2 --SiO.sub.2 core can advantageously be used as the amplifier fiber for EDFA, with the presence of Al considered to contribute to high efficiency of the amplification process. See, for instance, J. F. Massicott et al. (op. cit.), wherein a Al.sub.2 O.sub.3 --GeO.sub.2 --SiO.sub.2 fiber is disclosed. See also U.S. Pat. No. 4,923,279 to B. J. Ainslie et al. which discloses a silica-based EDFA fiber with refractive index difference 0.01 and with a core of overall diameter 4 .mu.m. The core consisted of an inner, Er-doped core region of diameter 1.5 .mu.m which also contained Si, P and Al, and an outer region that contained Si, Ge, and P. The deposited cladding of the fiber contained Si, P and F, with the P and F concentrations chosen such that the material had a refractive index equal to that of pure fused silica. The Al and Er distributions were apparently co-extensive. The fiber apparently was designed for relatively short .lambda..sub.p (528.7 nm) and apparently was not single mode at .lambda..sub.p.
Even though some general design principles are known for EDFA fibers, the prior art does not contain a design for low noise fibers of high efficiency and low amplification threshold. In view of the general importance of these characteristics and, in particular, their importance for a remotely pumped EDFA, it would be highly desirable to have available such designs, and in particular such designs that are manufacturable. Furthermore, it would be highly desirable to have available methods of making fiber that make it possible to attain previously unattainable parameter values. This application discloses both such designs and such methods.